Apparatus for polishing thin, flat semi-conductor wafers is well known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired.
More recently, chemical-mechanical polishing (CMP) apparatus have been employed in combination with a pneumatically actuated polishing head. CMP apparatus are used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A perspective view of a typical CMP apparatus is shown in FIG. 1A. The CMP apparatus 10 consists of a controlled mini-environment 12 and a control panel section 14. In the controlled mini-environment 12, typically four spindles 16, 18, 20, and a fourth spindle, (not shown) are mounted on a cross-head 24. On the bottom of each spindle, for instance, under the spindle 16, a polishing head 26 is mounted and rotated by a motor (not shown). A substrate such as a wafer is mounted on the polishing head 26 with the surface to be polished mounted in a face-down position (not shown). During a polishing operation, the polishing head 26 is moved longitudinally along the spindle 16 in a linear motion across the surface of a polishing pad 28. As shown in FIG. 1A, the polishing pad 28 is mounted on a polishing disc 30 rotated by a motor (not shown) in a direction opposite to the rotational direction of the polishing head 26.
Also shown in FIG. 1A is a conditioner arm 32 which is equipped with a rotating conditioner disc 34. The conditioner arm 32 pivots on its base 36 for the in-situ conditioning of the pad 38 during polishing. While three stations, each equipped with a polishing pad 28, 38 and 40 are shown, the fourth station is a head clean load/unload (HCLU) station utilized for the loading and unloading of wafers into and out of the polishing head. After a wafer is mounted into a polishing head in the fourth head cleaning load/unload station, the cross head 24 rotates 90° clockwise to move the wafer just loaded into a polishing position, i.e., over the polishing pad 28. Simultaneously, a polished wafer mounted on spindle 20 is moved into the head clean load/unload station for unloading.
A cross-sectional view of a polishing station 42 is shown in FIG. 1B. As shown in FIG. 1B, a rotating polishing head 26 which holds a wafer 44 is pressed onto an oppositely rotating polishing pad 28 mounted on a polishing disc 30 by adhesive means. The polishing pad 28 is pressed against the wafer surface 46 at a predetermined pressure. During polishing, a slurry 48 is dispensed in droplets onto the surface of the polishing pad 28 to effectuate the chemical mechanical removal of materials from the wafer surface 46.
During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. A slurry composition typically consists of an abrasive component, i.e, hard particles and components that chemically react with the surface of the substrate. For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10.
The polishing pad 28 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surface. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
Referring now to FIG. 2, a perspective view of a known CMP polishing station 42 is shown. The polishing station 42 includes a conditioning head 52, a polishing pad 28, and a slurry delivery arm 54 positioned over the polishing pad. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution on the top surface 60 of the polishing pad 28. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad during a polishing process. The surface grooves 64, while serving an important function of distributing the slurry, also presents a processing problem when the pad surface 60 gradually wears out after successive use.
The slurry delivery arm 54 shown in FIG. 2 delivers a slurry solution to the polishing pad 28 in a stationary manner. The distribution of the slurry solution over the top surface of the polishing pad depends on the rotation of the pad. Since the slurry solution is usually dispensed at the center of the polishing platen, i.e., at the center of the polishing pad, it is difficult to evenly spread the slurry solution over the pad surface by the rotation of the pad. As a result, the amount of slurry at the edge of the polishing pad is always less than that in the center region of the pad. This leads to a higher removal rate at the center of the pad when compared to the edge portion of the pad. And furthermore, a higher polishing noise level is made during the polishing process. The problem is more severe with newly designed polishing pads which have deeper surface grooves than older pads, and thus it becomes more difficult to spread the slurry solution uniformly on the polishing pad.